I put my 2019 in my 2020 amended tax returns in the same envelope they received my return but it's only showing the 2019 why is that

Yikes.  You're not supposed to do that.  They probably stapled it all together.  They only process 1 return per envelope.   You might need to mail it again.

Yep,  Mail room employees open the envelope and staple it all together and then send it tot the data entry clerks who enter the top tax return and consider anything else supporting information to be filed in the archives should it ever be needed by auditors.  These are all low paid employees who do a routine task 8 hours a day.